I have estimated stan_lm models using rstanarm.
One of the estimates is labelled R2.
There is also the bayes_R2.stanreg function.
Both the single R2 estimate (with mean, sd, percentiles, etc) and the bayes_R2 vector of estimates come from the posterior draws.
I don’t understand the distinction/purposes of the two R2’s.
I haven’t found similar questions in the forum.